1. Field of the Invention
The present invention relates to a phosphor which is coated with a protective layer and which can be used for a plasma display panel (PDP), and more particularly, to a phosphor used for a PDP and coated with a continuous, thin protective layer, thereby improving the optical characteristics and the lifetime of the PDP.
2. Description of the Related Art
Since a PDP self-radiates on a similar principle to a fluorescent lamp, an image shown on the PDP has a uniform brightness, a relatively high contrast, and a viewing angle greater than 160 degrees even if a screen of a PDP is large. The PDP is known to best display a large screen of 42–60 inches. In addition, a PDP has upper and lower glass plates and a thickness no greater than 10 cm even if a driving circuit is provided, and is very light as compared to other display devices. Therefore, PDPs have been considered as the next generation of display devices.
The gradation of a cathode-ray tube (CRT) is determined in accordance with the energy of an electron beam incident on the surface of the phosphor. In contrast, a PDP adjusts the gradation using the number of discharges. In other words, the display operation of a PDP is performed only by an ON/OFF of a discharge, and a discharge “ON” state can be implemented when the voltage is higher than a predetermined level.
In the case of PDPs, since the surface of the phosphor is exposed to a discharge and vacuum ultraviolet (VUV) rays under discharge conditions, the luminance, the luminescence, and the life characteristics decrease as time elapses. To overcome this problem, the surface of the phosphor is coated with a material, such as Al2O3 or Y2O3, having a wide band gap as a protective layer. However, most protective coatings are particulate coatings in which a protective material is discontinuously deposited on the surface of the phosphor in the form of particles, but not in the form of a continuous film. Such a particulate coating is known to greatly decrease the luminance. Particularly, in the case of the phosphor for a PDP, since an exciting source is a VUV ray (147 nm), the depth of penetration provoking the luminescence is only several hundred nanometers. Accordingly, such a decrease in luminance is great. Consequently, particulate coatings used at present do not satisfactorily serve as a protective layer and greatly decrease the optical characteristics so that their practical use is problematic.
To prevent the optical characteristics of the phosphor from decreasing due to a particulate coating and to improve the lifetime of the phosphor, a protective coating to efficiently protect the phosphor from sputtering that occurs during the discharge and the VUV rays is needed.
In addition, efforts to form a continuous coating layer on the surface of an individual phosphor particle have been made to improve the performance of a lamp for maintaining luminance. For example, U.S. Pat. Nos. 4,585,673, 4,710,674, and 4,825,124 disclose methods of depositing a continuous coating layer on a phosphor particle within a fluidized bed. However, these methods require a very sophisticated apparatus. Other methods of forming a continuous layer on phosphor are deep coating, vaporization, and drying methods that are disclosed in U.S. Pat. Nos. 2,151,496, 4,287,229, and 4,339,501. Particularly, U.S. Pat. No. 4,339,501 discloses a method of forming a continuous silica film by treating the phosphor for a CRT with a solution obtained by dissolving silica in an organic alkali solution.
In addition, U.S. Pat. No. 5,196,229 discloses a method for improving and maintaining the luminance of the phosphor for a fluorescent lamp by suspending phosphor particles in a solution containing a metal alkoxide, such as an isopropoxy or an ethoxy compound, which is then caused to gel into a gelled suspension. The gelled suspension is aged and/or dried to form a dried cake which can be readily broken up into powder. During the gelling and drying processes, the alkoxide hydrolyzes and polymerizes and reacts to form an oxide coating on each of the phosphor particles. However, U.S. Pat. No. 5,196,229 discloses forming a continuous coating film for a type of the phosphor, such as a type 4450 phosphor (for example, calcium fluorochlorophosphate activated by antimony and manganese), zinc silicate phosphor, and zinc sulfide EL phosphor, that is used only in fluorescent lamps.
Consequently, there is no known method of forming a continuous, protective coating on the surface of the phosphor used in a PDP, which has different luminescent characteristics than the phosphor used in a fluorescent lamp, where the coating is formed using a sol-gel process.